Direct methanol fuel cell with 3-D anode

ABSTRACT

Described is a direct oxidation liquid feed fuel cell having a PEM sandwiched between a cathode and an anode. The anode includes a reticulated structure having more than one path made of interconnected pores connecting each point on one face of the reticulate structure to points of its other side. In a preferred embodiment the anode fills at least part of the fuel tank.

This patent application derives priority from provisional patentapplication No. 60/579,603, incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to direct methanol fuel cells and to electrodesthereof.

BACKGROUND OF THE INVENTION

As portable consumer electronics become increasingly important, there isa strong demand for portable power sources with high energy density andwith the total power between several tenth of watt to a few watts. Upuntil now these demands are mostly met by different types of batteries.As a rule, these batteries are expensive, have a short operation lifeand also have disposal problems. Even the most advanced lithium ionbatteries are not able to meet the energy demands of modernsophisticated color displays, wireless access to the Internet,multiplayer games on cell—phones and tablet computers for note-taking,which all demand more power than earlier generations of electronicdevices did. For these new products, consumers want power sources thatlast days or weeks instead of hours.

Fuel cells that use hydrogen or methanol as fuel have considerableadvantages over batteries. Pure hydrogen has theoretical energy capacityof 32.8 kWh/kg. and methanol has energy capacity of 5.8 kWh/kg. Incomparison, Li—ion system, which is currently in use, has theoreticalcapacity of about 0.09 kW/kg.

Hydrogen solid polymer electrolyte fuel cells (PEM FC) are compact andhave a high power performance, which fits the requirements of modemportable electronics. Nevertheless there is a problem with PEM FCapplication in portable electronics field. Considering particularportable fuel cell applications, not only weight power density but alsovolume energy density is a very important parameter. Being a gaseousmatter, hydrogen has a very low theoretical volume energy density, about0.0029 kWh/1. That's why it is necessary to use some means to compactthe gas.

From the other side, methanol has a very high volume energy density,4.72 kW/1. Methanol is easy to obtain, store and transport. It is easilyavailable and inexpensive compound.

Nevertheless there is an unsolved problem with DMFC in portableelectronics applications. The problem is that currently DMFCs have lowspecific power. One way to increase DMFC specific power is to develop abetter anode catalyst. Up to now this way demonstrates only a limitedsuccess. The first relatively efficient catalyst for methanol oxidation(Pt—Ru catalyst) was introduced 30 years ago (see, for example, U.S.Pat. No. 4039409); from that time a big deal of improvements were madein a field of DMFC anode catalyst structure and in methods of thecatalyst integration into DMFC anode. Only small—scale improvements wereachieved to compare with “classic” Pt—Ru catalyst, whereas theseimprovements resulted in a substantial catalyst price increase.

SUMMARY OF THE INVENTION

The invention provides, according to a first aspect thereof, a directmethanol fuel cell with a 3-D anode, which is fully or in part combinedwith a fuel reservoir.

The invention also provides liquid feed fuel cell with an anodecomprising of reticulate body (also referred to herein as reticulatedstructure), which is electronically conductive and capable of catalyzingoxidation of said fuel.

The anode 3-D structure has catalyst disposed within the volume thereof.The catalyst is for catalyzing oxidation of the fuel, and it ispreferably affixed to the inner surface (also referred to herein asinner walls) of the 3-D anode. A direct methanol fuel cell according toone embodiment of the invention operates without requiring forced air orforced methanol flow and near room temperature. According to anotherembodiment, the fuel cell of the invention works with forced methanol,forced air flow, and/or at elevated temperature.

In a preferred embodiment, the electrochemical fuel cell includes asolid polymer electrolyte membrane, sandwiched between a cathode and ananode. The anode includes a 3-D structure, which also houses the fuel.

According to one embodiment air or oxygen may be used as oxidizer, andthe cathode includes gas diffusion layer, active catalyst layer andcathode current collecting layer. In other embodiments, fluid oxidizer,such as H₂O₂ may be used, and for this end, the cathode may have adifferent structure, which by itself is known in the art. According toone embodiment of the invention, the cathode also has 3-D electrode,with a structure similar to that of the 3-D anode, described in moredetails below.

The anode includes electronically conductive reticular foam, whichinternal surface has on it an oxidation catalyst, to allow thereticulated foam to serve as an anode. The foam serves also as a currentcollector.

In the present description and claims “reticulate structure” or“reticulate body” means a structure or body wherein there is more thanone way to travel within interconnected pores between points on one faceof the reticulate structure to points of its other side. The terms“pore” and “interconnected pores” as used herein cover also structuresthat are sometimes referred in the art as channels, bottle-necks, etc.

In a preferred embodiment, the solid polymer electrolyte membrane isplaced in a housing. The housing walls contain a gas outlet. Forinstance, the walls may include a membrane that is permeable to gas thatmay be generated in the housing in a course of the fuel cell operationbut not permeable to liquids.

The structure of the 3-D anode is reticular, and it has pores that aremuch larger (about 10 times or more) than the catalyst particles on theinner surface of the pores.

The anode structure should also be completely accessible for liquidfuel. That is to say that fuel travels freely from one side of the anodeto the other. The pores are preferably spaced in such a way that theymay all be reached from the outer surface of the anode, such that allparts of the inner anode surface is in contact with fuel, and fuel isnot trapped inside the 3-D anode. The pores may be located in anymanner: stochastic, ordered etc. The size of the pores should be largeenough as to allow free fuel flow through the anode. The required flowrate is to be determined in accordance with the current that the fuelcell is required to produce, where larger current requires larger flow.Nevertheless, when catalyst particles of conventional size are used(about 5 μm), this requirement is redundant with the requirement fromthe pores to be much larger than the catalyst particles.

The preferred parameters of the 3-D anode are pore size of from about100 μm to about 500 μm, preferably 200 μm; surface area of from about 30to about 100 cm²/cm³, preferably from about 60 to about 80 cm²/cm³;specific weight of from about 0.03 to about 0.06 g/cm³ and porosity offrom at least 50%, preferably from about 80% to about 97%.

A fuel cell of the invention is preferably fueled with methanol, or anyother liquid fuel known in the art as suitable for fuel cells, such asthose suggested in WO 01/5442. The oxidant is preferably oxygen or air,although a fuel cell according to the invention may also operate withcondensed phase oxidants, such as hydrogen peroxide.

To evaluate power density of a fuel cell according to the invention, itmay be assumed that a ¼ in. thick carbon foam sheet is used to design a3-D anode, and that this material has 100 pores per linear inch with aninternal area of 2000 square ft per cubic ft. In this case, the 3-Danode has an internal area of 42 cm² per each cm² of visible anodesurface. Also it may be assumed that the current density on the internalsurface of the 3-D anode is 8 mA/cm²; which is a current density,commonly achieved using “classic” DMFC anodes with commonly achievedovervoltage of about 0.25 V. The above-mentioned current density of 8 mAper sq. cm results in a current density of 42×8=336 mA per sq. cm ofvisible anode surface. Assuming DMFC over-voltage of 0.4 V for theentire cell, which is realistically low, this current gives specificpower of about 130 mW/cm². This value is several times higher thenspecific power achieved with state of the art DMFC.

BRIEF DESCRIPTION OF THE FIGURES

In order to understand the invention and to see how it may be carriedout in practice, a detailed description of a preferred embodiment willnow be given, by way of non-limiting example only, with reference to theaccompanying drawings, in which:

-   FIG. 1 is a schematic illustration of a DMFC according to one    embodiment of the invention;-   FIG. 2 is a schematic illustration of a portion of a 3-D anode    according to the invention; and-   FIG. 3 is a schematic illustration of a DMFC according to another    embodiment of the invention.

DETAILED DESCRIPTION THE DRAWINGS

FIG. 1 shows schematically a liquid feed fuel cell 10 according to theinvention. The DMFC 10 includes a cathode 12, a polymer electrolytemembrane (such as Nafion™) 14, and an anode 16, which is of areticulated electronically conductive body with catalyst on its innersurface. The anode 16 completely fills a fuel tank 17. The cathode 12and the anode 16 are each connected to its own current collectors 18 and20, respectively.

FIG. 2 is a schematic illustration of a portion 16′ of the anode 16. Theanode portion 16′ has a reticulate conductive body 22, defining pores24. The inner surface of the pores 24 is covered with catalyst particles26. The characteristic size of the pores 24 is much larger than thediameter of the particles 26. The anode portion 16′ is shown in FIG. 2to be attached to a portion 14′ of the PEM 14 of FIG. 1.

The conductive body 24 may be made of a conductive substance, such asmetal or carbon, or it may be made of an insulator, such as polyurethanefoam, coated with an electronically conductive coating. Such conductivecoating may be, for instance, conductive carbon black embedded in abinder.

FIG. 3 is a schematic illustration of a DMFC 30 according to anotherembodiment of the present invention. The DMFC 30 includes anair-breathing cathode 32, a Nafion™ polymeric electrolyte membrane (PEM)34, and an anode 35, which includes two parts: an on-membrane activelayer 36, and a 3-D anode portion 38. The fuel cell 30 is inside ahousing 40 having outlets for CO₂ in the form of gas permeable membranes42. The DMFC 30 also includes a fuel tank 44, partly filled with the 3-Danode portion 38.

The on-membrane active layer 36 includes catalyst particles (not shown).

Preferable catalytic particles for use on the active layer 36 or on thesurface of the 3-D anode portion 38 are made of high surface area carbongrains comprising small islands of catalytically active metals, metalalloys, or any other catalyst known in the art per se. Other preferredcatalyst particles are unsupported particles of platinum black, otherprecious metal black, precious metal alloy black, or any other catalystparticles known in the art per se. The 3-D portion 38 of the anode ismade of electronically conductive foam. Non-limiting examples ofsuitable foams are reticulated vitreous carbon foam; metal foam, andpolymeric or ceramic foam, which surface is covered with flexible orinflexible conductive layer. A sufficient quantity of oxidation catalystis to be presented on the conductive walls of the conductive foam. Thismay be accomplished by any suitable means, non-limiting examples ofwhich are electro-deposition of catalytically active metals or alloysonto the foam's surface, and application of ink with catalyst particlesto the anode internal surface. The 3-D anode portion 38 is affixed tothe “on-membrane” anode active layer 36. The 3-D anode portion 38function is both to support the catalyst and also to serve as a currentcollector; it also collects electrons from the “on-membrane” activelayer 36.

The cathode 32 is built as known-in the art air-breathing cathode. TheCathode 32 and the anode 36 are both in contact with the PEM 34. Thus,the DMFC 30 employs a membrane electrode assembly (MEA) comprising asolid ionomer or ion-exchange membrane disposed between two electrodes.

In operation, the oxygen (or air) moves through the porous cathodecurrent collector and GDL (gas diffusion layer) and is reduced at thecathode electro-catalyst layer, according to the equation (I)½O₂−2H⁺+2e⁻→H₂O   (I)At the anode, the fuel moves through the 3-D anode portion 38 and isoxidized on a catalyst, which is deposited on its walls, and also on theon-membrane active layer catalyst 36:CH₃ 0H+H₂O→CO₂+6e⁻+6H⁺  (II)The protons produced at the on-membrane active layer 36, travel throughPEM 34 towards the cathode 32. The protons produced at the 3-D anodeportion 38 travel through the fuel mixture (water mixed with fuel andmineral or organic acid), which fills the pores of the 3-D anode portion38, then through the active layer 36 and the PEM 34, towards the cathode32.

The electrodes are electrically coupled to each other through anexternal load to provide a path for electrons from the anode to thecathode. The invention is not restricted to implementation with theparticular fuel (methanol). The cell may be successfully used with otherorganic fuels, particularly with other alcohols, and particularly withethanol.

1. A direct methanol fuel cell (DMFC) with a 3-D anode, comprising aconductive reticulated body with catalyst particles on its innersurface.
 2. A DMFC according to claim 2, wherein said 3-D anode alsoincludes a non-reticulated conductive layer attached to a polymerelectrolyte membrane (PEM).
 3. A DMFC according to claim 1, wherein said3-D anode is combined with a fuel reservoir.
 4. A DMFC comprising asolid polymer electrolyte membrane (PEM), sandwiched between a cathodeand an anode; said anode comprising a 3-D structure, which on operationalso houses fuel.
 5. A DMFC according to claim 1, wherein the conductivereticulated body has pores with size of from 100 μm to 500 μm.
 6. A DMFCaccording to claim 1, wherein the 3-D anode has surface area of between30 and 100 cm²/cm³.
 7. A DMFC according to claim 1, wherein the 3-Danode has porosity of from 80% to 97%.
 8. A liquid feed fuel cellcomprising an anode with a reticulated body, which is electricallyconductive and capable of catalyzing oxidation of said fuel.
 9. A liquidfeed fuel cell according to claim 8, having catalyst particles on itsinternal surface.
 10. A liquid feed fuel cell according to claim 8,comprising a fuel tank, and said anode fills at least part of the fueltank.
 11. A liquid feed fuel cell according to claim 8, wherein saidreticulated body has pores with catalyst particles on their innersurface, and the pores are at least ten times larger than said catalystparticles.
 12. A liquid feed fuel cell according to claim 8, whereinsaid reticulate body is made of an insulator coated with a conductivecoating.
 13. A liquid feed fuel cell according to claim 12, wherein saidconductive coating also includes catalyst particles.
 14. A liquid feedfuel cell according to claim 8, wherein said reticulated body is atleast 50% void.
 15. A liquid feed fuel cell according to claim 8,wherein said reticulated body is completely accessible for fuel.
 16. Adirect oxidation liquid feed fuel cell having a PEM sandwiched between acathode and an anode, and said anode includes a reticulated structurehaving more than one path made of interconnected pores connecting eachpoint on one face of the reticulate structure to points of its otherside.